Mechanisms of 5‐hydroxytryptamine‐induced inhibition in the porcine myometrium

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Abstract
The present experiments were designed to clarify the mechanisms of the inhibitory response of 5-hydroxytryptamine (5-HT) in the porcine uterine circular muscle. Inhibitory responses induced by 5-HT (1 nM-1 microM) were not affected by apamin (1 microM), charybdotoxin (100 nM) or glibenclamide (20 microM) but were significantly attenuated by 4-aminopyridine (3 mM) and tetraethylammonium (3 mM). Imidazole (100 microM) decreased but 3-isobutyl-1-methylxanthine (30 microM), milrinone (30 microM) and Ro 20-1724 (10 and 30 microM) potentiated the 5-HT-induced inhibition. On the other hand, zaprinast (3-30 microM) had no significant effect on the inhibitory response of 5-HT. 5-HT caused a time (0-5 min)-and concentration (1 nM-1 microM)-dependent increase in the tissue cyclic AMP level, but had no effect on the tissue cyclic GMP level. A significant correlation (P < 0.05) was observed between the inhibition of contraction and tissue cyclic AMP level. The effect of 5-HT on contractile force and cytosolic Ca2+ level ([Ca2+]i) was investigated using fura-PE3-loaded myometrial strips. A low concentration of 5-HT (< or = 10 nM) inhibited the spontaneous contraction without changing the amplitude of the spontaneous [Ca2+]i increase, but a higher concentration of 5-HT (> or = 100 nM) decreased the resting [Ca2+]i and inhibited both the spontaneous [Ca2+]i increase and spontaneous contraction. High-K+ (50 mM) caused increases in muscle contractile force and [Ca2+]i. 5-HT concentrationdependently inhibited the high-K(+)-induced contraction (EC50, 45 nM) with only a small decrease in [Ca2+]i increase. Carbachol also caused increases in muscle contractile force and [Ca2+]i. 5-HT significantly decreased both the carbachol-induced contraction and [Ca2+]i increase, but was more potent at inhibition of contractile force than [Ca2+]i. In Ca(2+)-loaded myometrial strips, carbachol, but not caffeine, caused a transient increase in [Ca2+]i and contraction in the absence of external Ca2+ (EGTA, 1 mM). 5-HT inhibited both the carbachol-induced increases in [Ca2+]i release and contractile force. In the beta-escin permeabilized myometrium, 5-HT significantly inhibited the Ca(2+)-induced contraction. The present results indicate that 5-HT stimulates tissue cyclic AMP production, and inhibits the porcine uterine muscle contractility by a reduction in [Ca2+]i and in Ca2+ sensitivity of the contractile elements. Activation of K+ channels might be partially involved in 5-HT-induced inhibition of the myometrial contractility.

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